I have a multiband geotiff. I would like to set the color by the third band (singleband pseudocolor), and the opacity by the first band. Using the properties dialogue in QGIS, I have no trouble doing whatever I want with the color, however when I try to set the opacity to use the first band, or any particular band, the whole thing becomes completely transparent and/or doesn't render. This is in the 'transparency' tab of the layer properties dialogue. When the transparency band is set to 'none', I can add a value to the transparent pixel list, and things seem to work as expected, but with the third band, the one used for color.
Why do I not seem able to use a separate band to set the opacity? In the documentation for this feature (the transparency tab), the instructions say to set the band to 'none', but since there is a list, I should be able to select something else, right?
I'm using QGIS 2.14 on Xubuntu 16.04.